Magnetic disc drives may utilize a Flex Over (or On) Suspension (FOS) circuit to couple a preamp or the like to read-write head assemblies. An FOS assembly may be routed along an actuator assembly and may be supported on a read-write head suspension assembly. The FOS assembly may comprise a pair of electrically insulating plastic layers which encapsulate two separate pairs of conductors, which transmit write currents and read bias currents, to the write and read elements of the head assembly. The conductors may interconnect from a preamp to the read write heads. Transitory effects may exist for impedance becomes greater due to cracks in the conductors. FOS fault detection may be required to ensure a poor FOS connection does not exist in the head assembly. Writer open fault detectors, or the like, look for opens that are on the scale of kilohms (kΩs), while poor FOS connection-based faults may occur in the 20Ω to kΩs range.
In existing FOS fault detection, the FOS fault is detected, by way of example, approximately 8 ns after entering read mode and prior to degauss (i.e. after the read/write head slews from write current to degauss current, degauss is enabled and the FOS fault detection is performed) and when enabled, this fault delays the entire writer degauss function and reader turn-on by 3 ns in such an example, resulting in a 60% time penalty. This is demonstrated in FIG. 1, which is a diagrammatic waveform illustration of example prior art FOS fault detection 100. Therein, when a FOS fault is detected, and despite its immediate entry to the fault register, the fault is not reported until the following write cycle (mode) is entered. Thus, the previous complete write cycle 102 was potentially performed with an existing FOS fault 104 that is only detected at 106, during read cycle 108. There is an inherent sensitivity in existing FOS fault detection, such as when the end user opts to use a different degauss write current than what was used in the previous write cycle. The prior art waveform example of FIG. 1 shows an allotted time at 110 for this slewing to occur, and a magnitude delta between write and degauss currents, along with the polarity of slewing, leave a significant margin for error in existing FOS fault detection.